Group-III group-V compound semiconductors (often referred to as III-V compound semiconductors), such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices employing III-V compound semiconductors include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large band gap and high electron saturation velocity of many III-V compound semiconductors also make them excellent candidates for applications in high temperature and high-speed power electronics.
Epitaxially grown films of the III-V compound semiconductor GaN are widely used in the fabrication of light-emitting diodes. Unfortunately GaN epitaxial films must be grown on substrates other than GaN because it is extremely difficult to obtain GaN bulk crystals due to the high equilibrium pressure of nitrogen at the temperatures typically used to grow bulk crystals. Owing to the lack of feasible bulk growth methods for GaN substrates, GaN is commonly deposited epitaxially on dissimilar substrates such as silicon, SiC and sapphire (Al2O3). However, the growth of GaN films on dissimilar substrates is difficult because those substrates have lattice constants and thermal expansion coefficients different than those of GaN. If the difficulties of growing GaN films on silicon substrates could be overcome, silicon substrates would be attractive for GaN growth given their low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon substrates would also provide easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
Additionally, due to the lacking of appropriate substrates for growing GaN films thereon, the sizes of the GaN films are limited. The large stresses created by growing a GaN film on a dissimilar substrate may cause the substrate to bow. This bowing may cause several adverse effects. First, a great number of defects (dislocations) will be generated in the supposedly crystalline GaN films. Second, the thicknesses of the resulting GaN films will be less uniform; causing wavelength shifts of the light emitted by the optical devices formed on the GaN films. Third, cracks may be generated in large stressed GaN films.
The epitaxial lateral overgrowth (ELOG) technique has been used to form GaN films on dissimilar substrates in order to reduce stress and the number of dislocations in the film. FIGS. 1 and 2 illustrate a conventional ELOG process. Referring to FIG. 1, substrate 10 is provided. Under-layer 12, which comprises a nitride semiconductor (i.e., a III-V compound semiconductor in which the group V element is nitrogen), such as GaN, is formed on substrate 10. Dielectric masks 14 are then formed on under-layer 12. Next, a III-V compound semiconductor layer 16 is epitaxially grown, wherein the growth includes a vertical growth component and a lateral overgrowth component, which eventually results in a continuous III-V compound layer 16. In FIG. 2, an additional mask layer 18 is formed, followed by the growth of III-V compound layer 19. Again, the growth includes a vertical growth and a lateral growth, so that III-V compound layer 19 eventually becomes a continuous layer.
The III-V compound semiconductor film formation method shown in FIGS. 1 and 2 suffers from drawbacks. First, in the case substrate 10 comprises silicon, the silicon in the substrate may react with the nitrogen in under-layer 12 to form silicon nitride. The undesirably formed silicon nitride acts as an amorphous overcoat at the interface between silicon substrate 10 and under-layer 12. The amorphous overcoat adversely affects the film quality of the subsequently grown III-V compound semiconductor films. In addition, silicon nitride has a high resistivity, and hence prevents the formation of vertical optoelectronic devices, in which two contacts to the optoelectronic device are formed on opposite sides of substrate 10. New methods for forming III-V compound semiconductor films while overcoming the above-discussed drawbacks are thus needed.